Many valuable documents, such as, without limitation, bank notes, checks, identification documents, tickets, and postal indicia are produced and distributed/used in printed form. Unfortunately, various technologies are now readily available for making high-quality copies of such documents. As a result, falsification and counterfeiting of valuable documents is a serious concern.
One approach that has been developed to protect against falsification and counterfeiting of valuable documents has been to use a graphic security feature that is applied to or printed with the document to be protected. These features, often in the form of an image, are generally not viewable without the assistance of a specialized device that renders them visible. For example, one known graphic security method described in, for example, U.S. Pat. No. 7,512,280, involves adding a watermark image to material that is to be printed in the form of a slight shift in the position (from their normal position) of certain of the pixels (e.g., dots) of the material to be printed. The shifted pixels, which represent the watermark image, can only be rendered visible by applying a device to the document such as a transparent sheet made of many small lenses or a Moiré mask. Typical graphic security features that are specific for printed documents and that do not depend on special materials (special ink or paper), such as the watermarks and copy detection patterns (CDPs) just described, actually reside in the digital document to be printed (in other words, it is stored in the digital data that represents the document that is ultimately printed). As a result, it is possible to duplicate the digital document with its graphic security feature or to print it two or more times. This is a problem if the legitimate printed document is supposed to be unique. Particular applications where this problem is present include applications where documents, such as postal indicia, are printed at a customer site (using, for example, a digital postage meter).
Thus, there is at least a need for an improved system and method for protecting valuable digital documents from falsification and counterfeiting.